Wrap spring clutches function by controllably expanding or contracting the spring with respect to one or more surfaces. For example, a wrap spring can serve as the engaging element in a normally engaged overrunning clutch in which the input drives the output for only one direction of rotation, while permitting slippage in the other direction so that the output can remain stationary while the input continues to rotate. Following a change in direction from the slipping to the driving direction of rotation, it is inevitable that some amount of input rotation will take place before the output begins to be driven. This motion is called lost motion. There are three specific major causes for lost motion in an overrunning wrap spring clutch. (1) Clearances between component parts of the clutch are required for practical manufacture and assembly. Careful design and quality control can minimize but never eliminate lost motion due to these clearances. (2) The coil diameter of the spring must change in order to engage or release, as the case may be, the mating surface. In order for the diameter to change, it is obviously necessary that there be relative movement between the ends of the spring. This requires some amount of lost motion between the input and the output elements of the device, but the movement required to accomplish this change in spring diameter is extremely small and is usually negligible compared to other components of lost motion. (3) A potentially large component of lost motion can occur when a force is applied to an end of the spring for the purpose of changing its diameter. A force, applied at an end of the clutch spring in the direction such as to loosen the spring, will usually distort the final turn of the spring so that it moves away from the mating surface. This movement is the component of lost motion that is addressed by our invention. The first two components of lost motion are not addressed by our invention but are mentioned here simply for clarity.
Every method for controlling the engagement of a wrap spring clutch must, by some means, impart a bending moment to the spring to cause a change in the spring's radius of curvature. In some clutches, the means employed for changing the size of the spring is simply to apply a force that either pushes or pulls, as is appropriate, on the end of the spring. The moment results from the combination of the applied force together with reaction forces from other elements in the clutch. This method is used because it is mechanically simple, but it does not produce the ideal effect on the spring. The ideal method would be to apply a bending moment, or force couple to the end of the spring. A pure bending moment would change the diameter without producing any distortion. Unfortunately, it is usually not convenient to apply a bending moment directly to the end of a clutch spring, so a simple force is usually employed and the resulting lost motion is tolerated as a matter of practical necessity. Our invention provides a means for significantly reducing that distortion and eliminating much of the lost motion produced by the distortion.
The element that applies the force to the end of the spring can be a stationary stop with which the spring comes into contact as it rotates. Or it can be a surface on a coaxially mounted element which can contact the end of the spring.
Distortion in the ends of wrap springs has been analyzed by A. M. Wahl in the Journal of Applied Mechanics, Vol. 62, 1940, pp. A89-A91. Wahl's analysis applies to wrap springs as they are used in split-core clutches. These are clutches in which the clutch spring is wrapped over the adjoining portion of two concentric axially abutting cylindrical surfaces. This configuration produces a clutch whose input can drive the output for one direction of rotation, and overrun in the other. However, the distortion referred to in the Wahl analysis is largely eliminated in clutches in which the ends of the spring have been reformed after winding. Therefore the distortion discussed by Wahl is not an important factor in the clutches herein considered.
Our invention provides a means for eliminating the lost motion due to distortion of the last turn of the spring.